Found an AC 220V alternativeto North-American Kill-a-Watt on e-bay. It's a reworked original Kill-a-Watt actually. With the UK BS1363 socket Need to mount a Schuko one there, if you live in Germany e.g.

Any thoughts? There are not so many good power meters with 220V input.

I have one of these units. They are available in the UK from Maplin and I got mine on a special offer at Â£12.49.

Not sure on accuracy but the readings don't show much fluctuation with variation normally being 1W over time on the same load. It works well and has a kWh function which counts use over time. I've not used this function on a computer but it's been useful before on domestic applicances for counting cost over time and making a case for replacement based upon energy saving.

Has arrived for £15 from Amazon, and it can be found also on Ebay. It is completely the same device, from Maplin (hm, their server doesn't respond...) Had to pay some extra money to transfer purchase to Eastern Europe, but the device deserve the pay. Nothern American Kill-a-Watt (and this adapted euro-clone) is still the best power meter on mass market!

What is inside:

And some pics on how I made it Schuko compliant, lol It's just a little dumb box now, with some hanged wires

Sometimes you can't plug this thing into overpopulated socket set, or can't rich its buttons, or see screen readings, so I desided to attach a power cord to it. And a socket on cord for the output too, cause I couldn't find a small Schuko socket to integrate it into the case.

...

MoJo wrote:

Anyone in the UK should check Robert Dyas shops. They have them for £5 in the clearance section at the moment.

An article describing how to recalibrate the meter to measure small quantities of current, e.g. ten times lower. The meter's maximum admissable load rate decreases proportionally, but that's still bearable issue.

It's Brennenstuhl' PM230 on the photo, but Kill-a-Watt has the similar element (current sense bridge)

I have an NZR Standby-Energy-Monitor SEM 16+ which was pretty expensive (559 SEK = ~87$) and the specifications says 16A. I recommend this meter btw, it's very accurate on low loads. It is expensive though..

I don't know what is inside, I bought about ten for £1.80 each from Wilkinsons years back so never needed any more. The ones I have use fairly a standard power resistor and op-amp circuit, easy enough to interface.

There must be a specialized integrated circuit, not just op amps.Here is my meter EL-EPM02HQ (a good one):

It was a while ago when I opened it but basically there was an op-amp which did both current and voltage measurement outputs going into a chip-on-pcb blob which runs the display. I stuck a multimeter and then an oscilloscope on the op-amp and could take accurate measurements from it. Well, I say accurate, within 5% I think. I intend to hook an AVR up to the op-amp and let the PC monitor its own power usage, and will probably replace the resistor with one more suited to accurate measurement of <100W devices... Or maybe I should just do the whole thing from scratch.

Interesting! The ADE7755 outputs a square wave with frequency proportional to power consumption. Do the meters also show line voltage? In that case there must be other hardware measuring that.

In mine they used a dual op-amp. One to measures voltage by scaling approx ±600v to 0-5v, for more precisely 0-300v to 0-5v with the assumption that the negative part of the sine wave has the same amplitude at the positive part. The negative part is clamped. I bet that the input to the op-amp isn't full voltage though, more likely the output of a potential divider. I didn't check at the time.

The other one to measures current by scaling the voltage drop over the current sense resistor by some large factor I forgot. Frequency probably just uses a low logic level detector on the voltage output.

The ADE7755 provides some output isolation which is quite useful here. I bet the op-amp based ones have a high return rate.

At the moment I am trying to get a cheap analogue 'scope off eBay to do some mains work with. I don't want to risk my expensive digital one. In the UK the voltage is very stable, rarely deviating from 230V, so I may not bother measuring voltage at all. If you just assume that it is 230V RMS and a perfect sine wave (which over time it will be) then you only need to measure current and can use a hall effect sensor. I am thinking about doing a clock which uses mains frequency to keep time too because over a few hours it is extremely precise. Atmel have an app note where they connect an AVR pin to the mains via a 1M resistor and rely on the clamping diodes on the pin! It works but if how much do you trust that 1p resistor not to fail and pump 230v into your low voltage circuit?

Oh, a banal idea came into my mind! Concerning under 1-1,5W load measure.

It seems that lower operating limit of such devices can be extended simply by adding a known extra load (small, 2-5W), by connect a 10 kOhm resistor e.g., and it will result in 2W positive offset to meter readings

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